Review round: whether a recent library is there is kept apart from whether it can be
opened. `Unavailable::is_missing` (not found, a file, no library, a folder that can't be
listed) is the only answer that greys out File ▸ Open Recent Library and makes
`app.recentLibraries` say `exists: false`; an unreadable catalog or a failed check gives
`null`. A failed open from the launch window (say another program has the library)
drops the answer so it is checked again, instead of storing the open's error as one; the
launch list keeps the error on its row. Test: Open on a locked library from the launch
window, then the menu entry stays enabled and `exists` is true (fails if a failed open
marks it missing); an unreadable catalog gives `exists: null`. Docs worded to match.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
